1. Field of the Invention
The present invention generally relates to a radio unit, and in particular to an improvement of a planar antenna for radio apparatuses such as digital mobile telephones and other portable radio transceivers.
2. Description of the Related Art
A planar inverted-F antenna which can be miniaturized has been widely used in mobile communication apparatuses such as portable radio telephones. Since the frequency range which provides acceptable antenna gains is relatively narrow (generally, 4-5%), however, there have been proposed several antenna structures which can be used in a plurality of frequency bands or a wider frequency range. In an example of conventional antennas, two antennas having different resonance frequencies are used to provide two usably frequency bands. In another antenna, the volume of a element is doubled to substantially widen the frequency range.
Further, a patch antenna has been disclosed in Japanese Patent Unexamined publication No. 62-188504. This conventional antenna is provided with an adjuster for connecting two radiation elements or adjusting the amount of overlapped areas of the two radiation elements to achieve a wider frequency range where acceptable antenna gains are obtained.
However, the above conventional antennas need a plurality of radiation elements or the doubled volume of a radiation element. Such a large element cannot be suitable for mobile apparatuses such as portable telephones. On the other hand, the patch antenna needs a mechanical means for moving the radiation elements. Therefore, it is difficult to obtain a stable antenna characteristic and rapid switching of antenna frequency bands. Further, since the large amount of energy is required to move the radiation elements, the power consumption of a portable telephone is increased.